Distance Command
 Distance( <Point>, <Object> )

Yields the shortest distance between a point and an object.

Distance((2, 1), x^2 + (y  1)^2 = 1)
yields 1 
Distance((2, 1, 2), (1, 3, 0))
yields 3 
Let f be a function and A be a point.
Distance(A, f)
yields the distance between A and (x(A), f(x(A))).
The command works for points, segments, lines, conics, functions, and implicit curves. For functions, it uses a numerical algorithm which works better for polynomials. 
 Distance( <Line>, <Line> )

Yields the distance between two lines.

Distance(y = x + 3, y = x + 1)
yields 1.41 
Distance(y = 3x + 1, y = x + 1)
yields 0 
Let a: X = (4, 0, 0) + λ*(4, 3, 0) and b: X = (0, 0, 0) + λ*(0.8, 0.6, 0).
Distance(a, b)
yields 2.4
The distance between intersecting lines is 0. Thus, this command is only interesting for parallel lines. 
 Distance( <Plane>, <Plane> )

Yields the distance between the two planes.
Let eq1: x + y 2x = 1 and eq2: 2x + 2y +4z = 2. Distance(eq1, eq2)
yields 0.82
The distance between intersecting planes is 0. Thus, this command is only meaningful for parallel planes. 
See also Distance or Length tool . 